1. Field of the Invention
The present invention generally relates to application of an ionomer layer to a substrate, and more particularly to application of an ionomer layer to an electrode substrate or electrode suitable for use in an electrochemical fuel cell.
2. Description of the Related Art
Electrochemical fuel cells (“fuel cells”) have the capability of generating electricity from a fuel and an oxidant in a clean and efficient manner, and have been the focus of considerable attention. When hydrogen is used as the fuel, hydrogen is combined in the fuel cell with oxygen (in the air) to produce electricity, with heat and water being the only by-products. There are a number of types of fuel cells, distinguished largely by the type of electrolyte employed. One type of fuel cell utilizes a polymer electrolyte membrane (“PEM”), and is referred to as a PEM fuel cell. A PEM is an ion-exchange membrane comprising a solid, organic polymer that can conduct ionic species. The PEM that is useful for fuel cells is a proton exchange membrane that conducts protons and other cations, but not anions or electrons. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E. I. Du Pont de Nemours and Company under the trade designation Nafion®. The key component of a PEM fuel cell is the membrane electrode assembly (“MEA”), which is a consolidated assembly of an anode and a cathode (both typically planar), and a PEM sandwiched therebetween.
Each electrode (i.e., anode and cathode) comprises a substrate that is porous and electrically conductive. Such substrates are often prepared using woven or non-woven carbon paper or fabric that has been treated to impart a desired level of porosity and electrical conductivity. One side of each substrate is coated with a thin layer of catalyst, typically platinum. This catalyst layer is applied as a slurry of very small catalyst particles and ionomer particles suspended in a volatile solvent. Prior to application of the catalyst layer, a carbon sublayer is often applied to the substrate and dried in order to provide a suitable surface for application of the catalyst layer.
Following application of the catalyst slurry to the substrate, heat is applied to drive off the solvent. The ionomer particles bind the catalyst particles to form the catalyst layer, and further bind the catalyst layer to the substrate. If dried too quickly, the catalyst layer is prone to cracking and adhering poorly to the substrate. One approach used to counter this tendency is to dry in a staged fashion. In this manner, the substrate having the catalyst slurry applied thereto is exposed to more than one drying zone. The plurality of drying zones are characterized by different temperatures and airflows, where temperature and airflow can be incrementally varied from the initial to the final drying zone. Airflows used for drying can be adjusted and configured in various ways to increase drying rates in a staged fashion. As an example, roll support of the substrate web may be used in initial drying zones, while air flotation of the web is used in the final drying zones. A similar drying technique may be used for drying the carbon sublayer prior to application of the catalyst slurry.
In order to provide sufficient power output, MEAs are typically stacked to provide a “fuel cell stack”. Interposed between such stacked MEAs are separator or fluid flow field plates having channels. Hydrogen is directed to the electrochemically active area of the anode through these channels in the fluid flow field plate, while oxygen (e.g., in the air) is directed to the electrochemically active area of the cathode through different channels. The gaseous hydrogen diffuses through the porous anode substrate and contacts the catalyst layer, where it dissociates into protons and electrons. The PEM allows passage of the protons from the anode to the cathode, but not electrons. Instead, the electrons are conducted through the electrode substrate and separator plate (also called a current collector), and then through an external circuit to the cathode. Oxygen diffusing through the porous cathode substrate reacts at the catalyst layer with the protons and electrons to yield water and heat. In view of the diffusion that occurs through the anode and cathode substrates, the anode and cathode are also referred to as gas diffusion electrodes (“GDEs”) and their substrates as gas diffusion layers (“GDLs”).
The efficiency of this process and, thus, the power density (i.e., electrical power/volume of fuel cell) that is possible for a fuel cell is a function of, among other things, access to the PEM by the protons formed at the catalyst of the anode, as well as access of the protons passing through the PEM to the catalyst of the cathode. Thus, the physical structure of the MEA should provide minimal resistance to the movement of protons from the catalyst of the anode, through the PEM, and to the catalyst of the cathode.
For this reason, the anode and cathode are typically treated prior to assembly of the MEA. For example, when the PEM is made from Nafion®, and one major planar surface of each of the anode and cathode comprises a thin layer of platinum catalyst particles bound by Nafion®, an additional layer of Nafion® ionomer is often applied to the surface of each catalyst layer prior to assembly of the MEA. When the MEA is then formed by interposing the PEM between the two electrodes, followed by application of heat and pressure, a continuum of Nafion® ionomer is formed between the anode and cathode catalyst surfaces. This continuum is achieved by the ionomer of the PEM and in the catalyst layers, as well as the additional ionomer coating applied to the catalyst layers, melting together when heat is applied and then solidifying upon cooling.
There are, however, disadvantages associated with existing methods for application of this additional layer of ionomer over the thin catalyst layers on the electrode substrate. For example, one method for applying the additional layer of ionomer involves spraying a dispersion of ionomer particles onto the catalyst layer. Spraying results in considerable material waste due to poor transfer efficiency. Also, spraying can yield non-uniformities in the applied layer that can result in resistance to movement of protons. This is particularly problematic when the amount of material to be applied is very small. At workable processing speeds, spraying small amounts of material necessitates the use of low spray pressures, which often results in poor atomization and non-uniform spray patterns. This problem may be ameliorated by using intermittent spraying, and by adjusting the on and off times of a plurality of spray nozzles. However, this method also suffers from non-uniformity when only small amounts of material are applied to the surface, as well as variability associated with nozzle blockages and/or varying air pressure.
Another method of applying the additional layer of ionomer employs slot die coating a solution of ionomer. However, slot die coating typically yields a layer that is too thick, and that has excessive penetration of the ionomer into the substrate. Gravure coating and similar roll coating methods have also been used for this purpose. However, these methods involve direct contact between a gravure roll (or other roll) and the substrate surface having the layer of catalyst. This causes a number of problems. For example, the catalyst layer is quite fragile and its adhesion to the substrate is limited. Consequently, poorly adherent portions of the catalyst layer may be transferred to the roll coater, thus damaging the substrate surface and causing the loss of catalyst, as well as contaminating the roll which necessitates frequent cleaning.
Accordingly, there remains a need in the art for improved methods for adding an ionomer layer to the surface of a substrate, particular to the surface of an electrode substrate having a thin, fragile catalyst layer onto which the ionomer layer is to be applied. There is also a need in the art for products incorporating such substrates.